A typical tape transport system uses vacuum loop boxes to compensate for the inertial difference between the tape reels which supply and take up the tape and the capstan which actually moves the tape in the forward and reverse directions past the read/write head. The vacuum loop boxes provide extra lengths of the tape between the supply reel and the capstan and the capstan and the take-up reel while at the same time maintaining tension on the tape. The capstan which has a lower mass and inertia relative to the supply and take-up reels accelerates to normal running speed relatively fast compared to the time it takes the high mass supply reel and take-up reels to reach normal running speed. These extra lengths of tape in the loop boxes give the supply and take-up reels time to attain the running speed of the capstan without the tape being damaged or broken as would be the case if loop boxes were not used.
However, use of vacuum loop boxes implies the need of a control system for maintaining the tape loops within predetermined limits in the vacuum loop boxes during deviations in speed between the tape reels and the capstan.
A typical tape transport system employs analog methods to control reel speed and acceleration. Such a system requires tachometer feedback to permit the control to operate on the difference between desired and actual acceleration of the tape reel to return the tape to an equilibrium position within the loop box. Such a system is cumbersome and has a relatively slow response time necessitating relatively long vacuum loop boxes to compensate for the slow response time.
The present invention contemplates a reel servo control system which employs primarily digital techniques as opposed to analog techniques. Such as system eliminates the need for tachometer feedback and comparison of analog voltages with its attendant disadvantages of slow response times and larger vacuum loop boxes. Use of a reel servo control system which primarily utilizes digital techniques which minimize analog components provides more precise control of reel speed and acceleration with a savings in cost and space as well as power conservation at the reel drive motors.
More specifically, the present invention comprises a reel servo system wherein a current programmer provides discrete current levels indicative of required motor acceleration to a current comparator having values dependent on the position of the tape within the loop box to a current comparator. The current comparator also receives an input of the current level in the reel drive motor which is indicative of actual motor acceleration which is driven by voltage drive pulses of a duration determined by the difference between the current inputs to the current comparator. The difference between the input to the current comparator is indicative of the acceleration needed to return the tape loops to their appropriate positions in their respective loop boxes. This current difference is sampled at the end of each reel motor voltage drive pulse and dependent on the sign of the difference it increments or decrements an up-down counter until the current difference is virtually zero. The count in the up-down counter is fed into a pulse width counter which counts down from that count to zero which determines the time duration of the reel motor drive pulse. Thus, the length of the reel motor drive pulse, which may be all or part of the duty cycle, is determined by the count in the up-down counter which is dependent on the negative or positive difference between actual reel acceleration and desired reel acceleration.
The present invention also includes overriding circuitry which provides braking and plugging current to the reel drive motors when the capstan reverses direction to provide rapid reel deceleration to maintain the tape within required limits of its appropriate loop box.